1. Field of the Invention
This invention relates to a swing phase control for an artificial lower limb and to a prosthesis including the control.
2. Discussion of the Background
The use of hydraulic swing phase controls in artificial above knee limbs is well known. They commonly comprise a piston and cylinder assembly connected to the thigh and the shin part of the limb with the line of action of the control being offset from the center of rotation of the knee and with the two ends of the cylinder connected by variable orifices and check valves so that adjustment of the orifices changes the damping provided by the control and thereby modifies the swing phase behavior of the limb. However existing controls commonly have continuous fluid communication through narrow passages between the two ends of the cylinder, which results in there being too much resistance during those parts of the swing phase when no resistance is necessary. Furthermore, existing designs use orifices that operate with laminar flow so that their hydraulic resistance is inherently sensitive to changes in fluid temperature; thus changes in the swing phase characteristics of the limb occur when the fluid temperature changes. Moreover, such orifices have a linear relationship between pressure drop and flow so that their resistance rises linearly with the angular velocity of the shin whereas the amount of energy that has to be dissipated rises with the square of the angular velocity of the shin.
It is an object of this invention to provide a simplified swing phase control that also improves on the above characteristics.
According to the present invention there is provided a hydraulic swing phase control unit which in one embodiment provides minimal resistance to shin flexion until, at a predetermined angle of flexion which corresponds to the toe off position, it provides adjustable resistance to flexion, and in the reverse direction provides minimal resistance to extension until at another predetermined angle which is near to the fully extended position, it provides adjustable resistance to extension.
In a second embodiment of the invention there is provided a hydraulic swing phase control unit which provides minimal resistance to shin flexion until, at a predetermined angle of flexion which corresponds to the toe off position, it provides adjustable resistance to flexion, but at a further predetermined angle corresponding to the maximum angle of flexion that occurs during normal walking, it ceases providing resistance to flexion. It thereafter resists extension in the same manner described for the first embodiment. There is thus minimal resistance to either flexion or extension when the knee is in the position normally adopted for sitting or kneeling.
In both embodiments an adjustable sharp edged orifice is provided to adjust the resistance to flexion or to extension. It is well known that the characteristics of sharp edged orifices are relatively insensitive to changes in temperature. Furthermore their pressure drop increases in proportion to the square of the flow so that the resistance provided by the control rises with walking speed at the same rate as the amount of energy that has to be dissipated.
In both embodiments the internal reservoir pressure is maintained by a spring loaded rolling diaphragm although similar means such as a spring loaded piston or a bellows could perform the same function. Also, in both embodiments a spring return feature can be added to the control units.
It should be noted that the controls are suitable for use with either monocentric or polycentric knees. It should be further noted that the control units hereafter described are illustrated with their rod end uppermost and connected to the thigh whereas their effectiveness is unimpaired if the arrangement is inverted and the rod end is connected to the shin.
The control units may be connected with their effective line of action posterior to the knee axis in which case the unit retracts when the shin is flexed; or they may connected with their effective line of action anterior to the knee axis in which case the unit extends when the shin is flexed. It will be noted that these two ways of connecting the units result in different piston displacement and offset characteristics as the shin is flexed.
The control units may be arranged with either of the described embodiments combined with their line of action being either anterior or posterior to the knee axis. However for simplicity, but to still illustrate the principles involved, two combinations arising from the two embodiments and two thigh connection positions are hereafter described by way of reference to the following drawings.